CH. XIV] EFFECT OF APERTURE IN PROJECTION 617 



The illuminated aperture might be decreased by using a large 

 iris diaphragm to cover part of the condenser face a b. 



In the figure the aperture illuminated a'b', is less than the 

 diameter of the rear lens. If the size of the condenser were greatly 

 increased until its image was as large as the rear lens of the objec- 

 tive, the marginal ray from s, would move from sxa's' to szs'. 

 The entire aperture of the objective would be illuminated and no 

 more light would be used by a further increase in the size of the 

 condenser (Fig. 347). 



859. Image formation of a point not on the axis. Light from 

 t, will spread out over the angle w t y, which equals angle a tb, will 

 pass through a' b', and be collected to a point t', on the screen. 

 This light will of course fill a cone of which the limiting rays are 

 t w b' t' and t y a' t' (Fig. 347). 



860. Illumination of the screen image. Any single point on 

 the screen as s' or t', will be illuminated by light which has come 

 from the bright disc a' b'. The illumination will therefore depend 

 on the three factors, the brightness and area of the disc a 'b', and 

 its distance from the screen (Fig. 347). 



The area of the disc can, of course, be no greater than the area of 

 the back lens of the objective, and is usually smaller. For this 

 reason the brightest projection in a given case is obtained when the 

 back lens of the objective appears to be entirely filled with light. 



The brightness of this disc of light would, if it were not for light 

 losses, be exactly the same as that of the original source. This 

 follows from the fact that the brightness of an object remains the 

 same, except for light losses, when seen through a lens or a system 

 of lenses as when viewed directly. A lens can only change the 

 direction, not the intensity of light, or in other words it can only 

 change the apparent size of an object. 



This being the case the screen brightness is limited not by the 

 candle-power of a source but by its intrinsic brilliancy (candle- 

 power per square centimeter). This assumes the image of the 

 light to have an area great enough to cover the front lens of the 

 objective, which is the case with most microscopic projection. 



